We study the physics of condensed matter systems, usually the solid state, using first principles or ab initio methods. We solve the quantum mechanical many-body problem of interacting electrons and ions to the best of our abilities, with the fewest approximations possible, and with no adjustable parameters or fitting. The cost is that the calculations must be done numerically and can be quite difficult to perform. The advantage is that the results are generally reliable and accurate, and hence can be compared with confidence to experimental findings to help understand, clarify, and even predict observed physical phenomena.

The reliability also implies that we can use ab initio computer modeling as an ideal virtual laboratory to study microscopic physical phenomena. By ideal we mean that the physical setup (atomic positions, boundary conditions, imposed fields, etc.) is specifiable and modifiable at will.